Describe the usefulness of blue-white screening, also called ?-complementation, in cloning vectors such as pUC19. Include in your answer the terms polylinker, DNA ligase, lacZ gene, insertional inactivation, Xgal, and ?-galactosidase

What will be an ideal response?


Answer: This is a powerful screening method, because white colonies contain a gene inserted into their plasmid whereas blue colonies do not. In the example of pUC19, the lacZ gene contains a polylinker site where the plasmid can be digested and have a foreign gene cloned into the site. The plasmid is re-circularized by DNA ligase such that now the lacZ gene and therefore β-galactosidase (LacZ) activity is disrupted. When transformed cells are grown on a medium with the artificial Xgal substrate, all of those without an insert will have a functional LacZ and will convert the Xgal to a blue color. A white colony suggests the cells contain an inserted gene within the lacZ gene of the plasmid, and these white colonies are likely to be the ones of interest.

Biology & Microbiology

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A. Define a gene

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